Light emitting device having dopants in a light emitting layer

ABSTRACT

In order to improve the light emission efficiency and the light emission life time of an organic electroluminescent device, the invention provides a method for improving the dispersibility of a light emitting material in the light emitting layer. The light emitting device comprises electrodes consisting of an anode and a cathode provided on a substrate, and an organic light emitting layer between the electrodes, wherein the organic light emitting layer includes a light emitting material and a dopant for improving the dispersibility thereof. As the dopant, there are employed a light emitting compound and a non-light emitting compound or a current enhancing material. In case of employing the light emitting compound, the composition corresponds to a case of utilizing plural light emitting materials, and, in such case, the light emission wavelengths are preferably mutually closer. Also evaporation of the light emitting material and the dopant in a same evaporation boat allows to reduce the evaporation temperature, to improve the dispersibility of the light emitting material and to improve the device characteristics. In this manner there can be obtained a light emitting device of a high light emission efficiency and a long light emission life.

This application is a continuation of International Application No.PCT/JP02/05891, filed Jun. 13, 2002, which claims the benefit ofJapanese Patent Application Nos. 181416/2001, filed Jun. 15, 2001,143441/2002, filed May 17, 2002, 143442/2002, filed May 17, 2002 and143443/2002, filed May 17, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device utilizing anorganic compound, and more detailedly to a light emitting device,particularly an organic electroluminescent device (organic EL device),having excellent luminance, efficiency and drive durability by doping alight emitting layer with plural compounds.

2. Related Background Art

The organic EL device is being actively investigated for itsapplications as a light emitting device capable of showing a high speedresponse and a high efficiency. The basic configuration of such deviceis shown in FIGS. 1A, 1B and 1C (for example see. Macromol. Symp., 125,1-48 (1997)).

As shown in FIGS. 1A, 1B and 1C, the organic EL device is generallycomposed, on a transparent substrate 15, of a transparent electrode 14,a metal electrode 11, and an organic layer sandwiched therebetween andconsisting of plural organic films.

In the configuration shown in FIG. 1A, the organic layer consists of alight emitting layer 12 and a hole transport layer 13. The transparentelectrode 14 is composed for example of ITO having a large workfunction, thereby achieving satisfactory hall injection characteristicsfrom the transparent electrode 14 into the hole transport layer 13. Themetal electrode 11 is composed of a metallic material of a small workfunction such as aluminum, magnesium or an alloy thereof for achievingsatisfactory electron injection characteristics into the light emittinglayer 12. These electrodes have a film thickness of 50 to 200 nm.

In the light emitting layer 12, there is employed for example analuminum quinolinol complex having electron transporting property andlight emitting characteristics (as exemplified by Alq3 shown in thefollowing). Also in the hole transport layer 13, there is employed amaterial showing electron donating property such as a biphenyl diaminederivative (as exemplified by α-NPD shown in the following).

The device of the above-described configuration shows an electricrectifying property, and, when an electric field is applied in such amanner that the metal electrode 11 becomes a cathode and the transparentelectrode 14 becomes an anode, the electrons are injected from the metalelectrode 11 into the light emitting layer 12 and the holes are injectedfrom the transparent electrode 14 into the light emitting layer 12through the hole transport layer 13.

The injected holes and electrons cause recombination in the lightemitting layer 12 to generate excitons, thereby generating lightemission. In this operation, the hole transport layer 13 serves as anelectron blocking layer, whereby the efficiency of recombination isincreased at the interface of the light emitting layer 12 and the holetransport layer 13 thereby improving the light emitting efficiency.

In the configuration shown in FIG. 1B, an electron transport layer 16 isprovided between the metal electrode 11 and the light emitting layer 12in FIG. 1A. Such configuration separates the light emission from thetransportation of electrons and holes, thereby achieving more efficientcarrier blocking and realizing efficient light emission. As the electrontransport layer 16, there can be employed, for example, an oxadiazolederivative.

Conventionally, the light emission in the organic EL device is generallybased on the fluorescence of molecules of a high emission center in ashift from a singlet exciton state to a base state. On the other hand,there is being investigated a device utilizing phosphorescence through atriplet exciton state, instead of the fluorescence through the singletexciton state. Representative examples of the references reporting suchdevice are:

-   1) D. F. O'Brien et al, Improved Energy Transfer In    Electrophosphorescent Device, Applied Physics Letters Vol. 74, No.    3, p. 422 (1999), and-   2) M. A. Baldo et al, Very High-efficiency Green Organic    Light-emitting Devices Based On Electrophosphorescence, Applied    Physics Letters, Vol. 75, No. 1, p. 4 (1999).

In these references, there is principally employed an organic layer of a4-layered configuration as shown in FIG. 1C, consisting of a holetransport layer 13, a light emitting layer 12, an exciton diffusionpreventing layer 17 and an electron transport layer 16 from the anodeside. There are employed following carrier transporting materials andphosphorescence emitting materials, which are abbreviated as follows:

Alq3: aluminum-quinolinol complex

α-NPD: N4,N4′-dinaphthalen-1-yl-N4,N4′-dipheny-biphenyl-4,4′-diamine

CBP: 4,4′-N,N′-dicarbazole-biphenyl

BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline

PtOEP: platinum-octaethylporphilline complex

Ir(ppy)₃: iridium-phenylpyridine complex

Also Forrest et al., Nature, 403, p. 750 discloses an EL device oflaminated structure utilizing CBP as a host material of the lightemitting layer, and causing triplet-singlet energy transfer from a greenlight emitting layer based on Ir(ppy))₃ to a red light emitting layerbased on DCM (dicyanomethylene).

These configurations are different from that of the present invention inthat the co-existing light emitting materials have distant lightemitting wavelengths and that the forming method does not involve vacuumevaporation of a mixture, as will be clarified later in the examples.

In the above-described organic EL device utilizing phosphorescent lightemission, it is important to inject a larger amount of carriers into thelight emitting layer at a lower voltage while maintaining the balance ofelectrons and positive holes at such lower voltage, in order to achievea high luminance and a high efficiency.

Among such phosphorescent materials, there are known ones with lowcharge injecting and charge transporting properties, in which it isdifficult to cause a large current at a low voltage.

Also many organic materials are known to form a cluster of pluralmolecules at the evaporation, and the light emitting layer involvingsuch clusters is considered to show a locally high concentration of thelight emitting material, leading to a loss in the light emittingefficiency of the device.

Also the organic materials are known to cause deterioration of thecharacteristics, for example by crystallization of the same molecules inthe light emitting layer.

Because of the above-described background, there is desired a lightemitting device capable of providing a high luminance of light emissionand a long service life.

SUMMARY OF THE INVENTION

In consideration of the drawbacks in the conventional technologiesexplained in the foregoing, the object of the present invention is toprovide an organic EL device utilizing an organic light emittingmaterial, enabling low-voltage drive and achieving a high luminance, ahigh efficiency and a high durability.

The above-mentioned object can be attained, according to the presentinvention, by a light emitting device provided with electrodesconsisting of an anode and a cathode formed on a substrate and anorganic light emitting layer between such electrodes, the device beingfeatured in that the aforementioned light emitting layer contains alight emitting material and a dopant for improving the dispersibilitythereof.

The light emitting device of the present invention is also featured inthat the aforementioned dopant is composed of a light emitting compound,and that the light emission spectrum of the aforementioned lightemitting material and that of the light emitting compound mutuallyoverlap in a principal portion.

The relationship between the light emission wavelength and the quantumyield of the aforementioned light emitting material and theaforementioned light emitting compound is preferably such that thequantum yield of either having a shorter light emission wavelength islarger than that of the other having a longer light emission wavelength.

At least either of the aforementioned light emitting material and theaforementioned light emitting compound is preferably a metal complexand/or an organic compound, and they preferably have respectivelydifferent HOMO levels.

The difference in the peak wavelengths of the light emission spectra ofsuch light emitting material and light emitting compound preferably doesnot exceed 30 nm.

The aforementioned light emitting material and light emitting compoundare preferably composed of plural metal complexes having a same ligandskeletal structure with respectively different substituents in suchligand skeleton, and the central metal of the metal complexes ispreferably iridium.

The present invention is also featured by a producing method in whichthe light emitting material and the light emitting compound are mixedand are subjected to vacuum evaporation in a single heating container.

The light emitting device of the present invention is also featured inthat the aforementioned dopant is composed of a non-light emittingcompound.

Such non-light emitting compound preferably has a boiling point lowerthan that of the aforementioned light emitting material.

Also such non-light emitting compound preferably has a band gap largerthan that of the light emitting material.

The proportion of the light emitting material and the non-light emittingcompound in the organic light emitting layer is preferably changeddepending on the position therein. The light emitting material ispreferably a phosphorescent light emitting material in terms of thelight emitting efficiency.

The light emitting device of the present invention is further featuredin that the organic light emitting layer contains a light emittingmaterial and a current enhancing material.

Preferably such current enhancing material is composed of a lightemitting material and has a quantum yield lower than that of theaforementioned light emitting material, and the difference of the peakwavelengths in the light emission spectra of these materials preferablydoes not exceed 30 nm.

The aforementioned current enhancing material has a band gap larger thanthat of the aforementioned light emitting material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are schematic views showing examples of theconfiguration of the light emitting device of the present invention,wherein FIG. 1A shows a device configuration with a two-layered organiclayer, FIG. 1B shows a device configuration with a three-layered organiclayer, and FIG. 1C shows a device configuration with a four-layeredorganic layer;

FIG. 2 is a chart showing light emission spectra of the light emittingmaterial and the light emitting compound employed in the presentinvention, with the abscissa representing wavelength and the ordinaterepresenting normalized intensity of light emission, illustrating anexample of an Ir complex C and an Ir complex D and showing a fact thatthe light emission spectra mutually overlap in a principal portion; and

FIG. 3 is a chart showing light emission spectra of a reference example,indicating that the light emission spectra mutually overlap less andhave mutually distant peak wavelengths.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The light emitting device of the present invention is provided with ananode, a cathode, and an organic light emitting layer sandwiched betweenthe anode and the cathode. The organic light emitting layer is notparticularly limited in configuration, and may assume configurations asshown in FIGS. 1A, 1B and 1C.

The light emitting device of the present invention is featured in thatthe organic light emitting layer contains a light emitting material anda dopant for improving the dispersibility of the light emittingmaterial, particularly a light emitting compound, or a non-lightemitting compound, or a current enhancing material. The aforementioneddopant provides various improvements such as:

(1) increasing the device current even in a device with a light emittinglayer in which the carrier injection or carrier movement is difficult,for example a light emitting layer utilizing a phosphorescent lightemitting material, thereby achieving a decrease in the drive voltage ora higher light emitting efficiency;

(2) suppressing the crystallization in the light emitting layer, therebyextending the service life of the device;

(3) reducing the evaporation temperature by co-evaporation with thelight emitting material; and

(4) changing the light emitting position within the light emittinglayer, thereby achieving an increase in the luminance etc.

The light emitting device of the present invention is featured, incomparison with a light emitting device having a similar configurationexcept for the organic light emitting layer, by a larger current or ahigher light emission luminance under the application of a similarvoltage, or a longer durability in continuous drive, because of a factthat the organic light emitting layer is not constituted by a singlelight emitting material but is a mixed light emitting layer consistingof a light emitting material and a light emitting compound.

The dopant, in case of a light emitting compound, preferably has a lightemission quantum yield lower than that of the principal light emittingmaterial. In this manner the principal light emitting materialrepresents a major portion of the light emission luminance while thecontribution of the dopant or the light emitting compound to the lightemission luminance can be made smaller.

A second function of the dopant is to stabilize the light emittingmaterial present in the light emitting layer. In such function, thelight emitting compound preferably has a molecular structure differentfrom that of the light emitting material and capable of inhibiting thecrystallization or dimerization in the base state or the formation of anassociate in the excited state. The light emitting material and thelight emitting compound are desirably similar in the light emittingproperty but are different in the molecular structure, for example, incase of metal complexes, having a same basic skeletal structure butbeing different in the substituents.

A third function of the dopant is to control the molecular flow at theevaporation. The evaporation under heating of a mixture of pluralmaterials of different evaporation temperatures allows to suppress theformation of a cluster such as microcrystals. Such effect can beexpected for example by evaporating a fluorinated organic compoundtogether with the light emitting material.

For example following compounds can be conceived as a fluorinated ligandof iridium complex:

In case a light emitting compound is employed as the dopant, it isimportant to obtain light emission as close as possible to a singlecolor. It is therefore preferred that the light emission spectrum ofsuch light emitting compound overlaps with that of the light emittingmaterial in a principal portion, or that the difference of the peakwavelengths in the light emission spectra of the two does not exceed 30nm.

For example, in case the light emitting material emits red light and theintensity ratio of the light emission of the light emitting material andthe light emitting compound is 10:1, it is confirmed by a simulationthat the CIE coordinate value of the emitted light does not changesignificantly if the difference of the peak wavelengths in the lightemission spectra is 30 nm or less. Therefore, from the standpoint ofobtaining light emission of a high color saturation, it is preferredthat the difference of the peak wavelengths in the light emissionspectra of the light emitting material and the light emitting compounddoes not exceed 30 nm.

In this manner there can be obtained a device showing little change inthe color saturation even when light is emitted from both the lightemitting material and the light emitting compound. Also, in case anenergy transfer is involved from the light emitting compound to thelight emitting material, there can be obtained an advantage offacilitating such energy transfer because of the small energydifference.

Also by selecting the band gap of the light emitting compound largerthan that of the light emitting material, the recombination of electronand positive hole tends to take place easier on the light emittingmaterial than on the light emitting compound, whereby the light emissioncan be obtained principally from the light emitting material.

In the present invention, the proportion of the light emitting materialand the light emitting compound may be varied depending on the locationwithin the organic light emitting layer, thereby controlling thedistribution of the electrons and the positive holes within the lightemitting layer, and regulating the position of the electron-positivehole recombination within the light emitting layer. In this manner therecan be prepared a device of high efficiency with satisfactory lightemission color.

In the present invention, the non-light emitting compound means acompound which is significantly inferior to the aforementioned lightemitting compound in the light emitting property and does not emitelectroluminescent light singly, thus not contributing to the lightemission of the EL device.

The light emitting layer is generally composed of the light emittingmaterial dispersed in a host material having electroconductivity, butcan also be composed of the light emitting material only. The presentinvention is featured in that a dopant is further added to suchmaterials. The host material can be, for example, CBP or TAZ, and thelight emitting compound employable in the present invention can be, forexample, the compound A shown below, CBP or Ir complex A.

The light emitting material can be, for example, Ir complex B, Ir(ppy)₃,or Ir complex C.

In the following there are shown the structures of other compounds, inaddition to those mentioned in the foregoing:

EXAMPLES

At first there will be explained a common part of the device preparationprocesses employed in the examples 1 and 2.

In these examples, there was employed a device configuration with afour-layered organic layer as shown in FIG. 1C. An ITO film (transparentelectrode 14) of a thickness of 100 nm was patterned on a glasssubstrate (transparent substrate 15). On thus prepared ITO substrate,following organic layers and electrode layers were formed in successionby vacuum evaporation by resistance heating in a vacuum chamber of 10⁻⁴Pa:

hole transport layer 13 (40 nm): α-NPD

light emitting layer 12 (40 nm): host material+light emittingmaterial+light emitting compound

exciton diffusion preventing layer 17 (10 nm): BCP

electron transport layer 16 (30 nm): Alq3

metal electrode layer 1 (15 nm): AlLi alloy (Li content 1.8 wt. %)

metal electrode layer 2 (100 nm): Al

These layers were so patterned that the electrodes have an opposed areaof 3 mm².

Example 1

A device was prepared by employing CBP as the host material of a lightemitting layer and doping the light emitting layer with the Ir complex Cas the light emitting material at a concentration of 7 wt. % and withthe Ir complex A as the light emitting compound at a concentration of 3wt. %. The employed Ir complex A has a function of increasing thecurrent in the device, thus also serving as the current enhancingmaterial.

Comparative Example 11

A device was prepared as in the example 1, except that the doping withthe Ir complex A as the light emitting compound was not executed.

Table 1 shows the results of measurement of current and luminance ofthese devices under the application of a DC voltage of 10 V.

TABLE 1 Current (mA/cm²) Luminance (cd/m²) Example 1 80.2 806 Comp. Ex.11 11.8 426

The example 1, utilizing the Ir complex A as the light emittingcompound, showed increases in the current and in the luminance. Also thecurrent enhancing effect of the Ir complex A could be confirmed from asignificant increase in the device current.

The light emission spectrum shows not only the light emission from theIr complex C but also from the Ir complex A. The Ir complex C has alight emission spectrum having a peak at 620 nm, while the Ir complex Ahas a light emission spectrum having a peak at 610 nm. Since thedifference of the peak wavelengths in the light emission spectra did notexceed 30 nm, the value on the CIE coordinates did not show anysignificant change.

The complex A had a quantum yield of 0.3 while the Ir complex C had aquantum yield of 0.66. The quantum yield was determined in the followingmanner:

Φ(sample)/Φ(st)=[Sem(sample)/Iabs(sample)]/[Sem(st)/Iabs(st)]

Φ(sample): quantum yield of measured sample

Φ(st): quantum yield of standard substance

Iabs(st): absorption coefficient at excitation wavelength of standardsubstance

Sem(st): area intensity of light emission spectrum of standard substancewhen excited at the same wavelength

Iabs(sample): absorption coefficient at excitation wavelength ofmeasured sample

Sem(sample): area intensity of light emission spectrum of the measuredsample when excited at the same wavelength.

The quantum yield Φ mentioned herein is represented by a relative value,taking Φ of the Ir complex G (to be explained later) as unity. Also Iabswas measured with a UV spectrophotometer (Shimadzu Mfg. Co.: UV3100),and Sem was measured with a fluorescent spectrophotometer (Hitachi Co.:F4500).

Example 2

A device was prepared by employing CBP as the host material of the lightemitting layer, and doping the region of a thickness of 10 nm at theside of the hole transport layer within the light emitting layer of athickness of 40 nm, with the Ir complex C as the light emitting materialat a concentration of 7 wt. % and with the Ir complex A as the lightemitting compound at a concentration of 3 wt. %, while co-evaporatingthe Ir complex C alone at a concentration of 7 wt. % in the remaining 30nm region. The employed Ir complex A has a function of increasing thecurrent in the device, thus also serving as the current enhancingmaterial.

Table 2 shows the results of measurement of current and luminance of theabove-described device and the device of the comparative example 11under the application of a DC voltage of 10 V.

TABLE 2 Current (mA/cm²) Luminance (cd/m²) Example 2 23.5 621 Comp. Ex.11 11.8 426

The results in Table 2 confirm that the device of the example 2 showedan increase in the current and the luminance in comparison with thedevice of the comparative example 11, and that the light emittingcompound, even in case of doping only a part of the light emittinglayer, has an effect of increasing the current and the luminance.Following Tab. 3 shows the quantum yield and band gap of the Ir complexA and the Ir complex C. The Ir complex A has a larger band gap and asmaller quantum yield.

TABLE 3 Quantum yield Band gap Ir complex A 0.3 2.02 eV Ir complex C0.66   2 eV

In the comparison of the examples 1 and 2, the light emission spectrumof the example 2 showed a weaker light emission from the Ir complex Aand a higher proportion of the light emission from the Ir complex C incomparison with the example 1. This is because the injection of positiveholes became easier by the current enhancing effect, whereby theelectron-positive hole recombination and the light emission principallytook place in the Ir complex C.

Example 3

In this example, there was employed a device configuration with afour-layered organic layer as shown in FIG. 1C. An ITO film (transparentelectrode 14) of a thickness of 100 nm was patterned on a glasssubstrate (transparent substrate 15) On thus prepared ITO substrate,following organic layers and electrode layers were formed in successionby vacuum evaporation by resistance heating in a vacuum chamber of 10⁻⁴Pa:

Hole transport layer 13 (40 nm): FL03 (following chemical formula)

Light emitting layer 12 (40 nm): host material+light emitting material1+light emitting material 2 Electron transport layer 17 (50 nm): Bphen(following chemical formula)

Electron injection layer 16 (1 nm): KFMetal electrode layer (100 nm): AlIt was so patterned that the electrodes had an opposed area of 3 mm².

In forming the light emitting layer 12, the Ir complex C was employed asthe light emitting material 1, and the Ir complex D was employed as thelight emitting material 2.

The Ir complex C and the Ir complex D were measured in equal amounts andwere agitated and mixed under crushing of the crystals in an agatemortar to obtain powder mixture.

Thus obtained powder mixture was charged in an evaporation boat and wassubjected to co-evaporation with CBP as the host material. Theco-evaporation with the host material was conducted in such a mannerthat the aforementioned mixture of the Ir complex C and the Ir complex Drepresented 7 wt. %.

The characteristics of thus prepared device are shown in the followingtable.

Comparative Example 31

A device was prepared utilizing only the Ir complex C of the lightemitting material 1 as the light emitting material.

Comparative Example 32

A device was prepared utilizing only the Ir complex D of the lightemitting material 2 as the light emitting material.

The results of evaluation of these devices are also shown in thefollowing table.

TABLE 4 Characteristics Luminance at luminance half-life 100 cd/m² (hr)from Light volt- current power initial emitting age efficiencyefficiency value material (v) (cd/A) (lm/W) 1000 cd/m² Example 3 Ircomplex C + 5.7 13.5 7.6 52 Ir complex D Comp. Ir complex C 5 7.6 4.8 50Ex. 31 Comp. Ir complex D 7.5 6.8 2.9 5.4 Ex. 32

In the device of the present example, the drive voltage required forlight emission at 100 cd/m² was 5.7 V, which was somewhat higher than 5V in the comparative example 31 but was significantly lower than 7.5 Vin the comparative example 32.

Also the current efficiency (measured in cd/A) was 13.5 cd/A, which wassignificantly higher than 7.6 cd/A in the comparative example 31 and 6.8cd/A in the comparative example 32.

The situation was similar also in the power efficiency, and the deviceof the present example was very efficient with a power efficiency of7.61 m/W which is significantly higher than 4.81 m/W in the comparativeexample 31 and 2.91 m/W in the comparative example 32.

Furthermore, the half life of the luminance in the continuous drive ofthe device from an initial luminance of 1000 cd/m² was 52 hours,corresponding to a significant improvement in comparison with 50 hoursin the comparative example 31 and 5.4 hours in the comparative example32. The half life of the luminance attained an improvement of more than10 times in comparison with the comparative example 32, and isconsidered to represent a particularly large effect of the presentinvention.

In the present example, the Ir complex C has a quantum yield of 0.66while the Ir complex D has a quantum yield of 0.92. The peak wavelengthof light emission is 620 nm in the Ir complex C and 595 nm in the Ircomplex D.

However, in case of employing an Ir complex I having a peak wavelengthof light emission of 595 nm same as in the Ir complex D but having alower quantum yield of 0.29, the light emission efficiency at theluminance of 300 cd/m² and the half life were inferior to those in thedevice employing the Ir complex D. It is therefore found desirable thatthe quantum yield of the light emitting material having a shorterwavelength of light emission is larger than that of the light emittingmaterial having a longer wavelength of light emission.

TABLE 5 Light emitting Efficiency Half life material (300 cd/m²) cd/A(hours) Ir complex C + 11.1 93 Ir complex D Ir complex C + 8.3 18.2 Ircomplex I Ir complex I

In the iridium complexes employed in the present example, the levels ofthe highest occupied molecular orbit (HOMO) and of the lowest unoccupiedmolecular orbit (LUMO) were as follows.

TABLE 6 Ir complex C Ir complex D HOMO (eV) −5.13 −5.32 LUMO (eV) −2.47−2.6

Both the HOMO level and the LUMO level were higher in the iridiumcomplex C than in the iridium complex D.

The electron levels were determined, based on the measurement ofoxidation-reduction potential by cyclic voltammetry (model:Electrochemical Interface SI 1287, Solartron Inc.) and the data of bandgap measurement by optical absorption, by conversion with reference tothe separately measured HOMO of the Ir complex C (model: AC-1, RikenKiki Co.).

Then, FIG. 2 shows the photoluminescence (optically excited lightemission spectra in dilute toluene solution) of the Ir complex C and theIr complex D employed in the present example. The light emission spectraof these two compounds are mutually very close and mutually overlap inthe principal portion of the spectra. The shift in color is notconspicuous because of the use of the light emitting materials havingvery close light emission wavelengths.

The Ir complex C and the Ir complex D, employed in the present example,have the evaporation temperature in vacuum of 267° C. and 234° C.respectively, and, in general, the evaporation temperature is lower inthe iridium complex including fluorine atoms. One of the features of thepresent invention lies in a fact that the molecular flow in theevaporation process can be controlled (for example control of clustersize) by evaporating a mixture of light emitting materials of differentevaporation temperatures from a same crucible.

Also the electric current supplied to the heating container, requiredfor evaporation of the present example, was lower in the mixture,indicating a less thermal impact at the evaporation. These results areshown in the following table.

TABLE 7 Efficiency Boat current Light emitting material (cd/A) (Amp) Ircomplex C 6.5 56.1 Ir complex C + Ir complex D 12.8 53.7

Comparative Example 33

Following table shows comparison with a case of evaporating the Ircomplex C and the Ir complex D from different boats.

TABLE 8 Evaporation Characteristics at luminance 100 cd/m² of lightCurrent Power emitting efficiency efficiency material Voltage (V) (cd/A)(lm/W) Example 3 same boat 5.7 13.5 7.6 Comp. ex. 33 different 5.5 9.65.8 boats Comp. ex. 31 Ir complex 5 7.6 4.8 C only

In comparison with the case of evaporating the Ir complex C only, thecurrent efficiency and the power efficiency were improved even in caseof forming the mixed light emitting layer with the Ir complex D byevaporation from different boats. On the other hand, in the presentexample 3, in which the two complexes are mixed and evaporated from asame boat, the current efficiency and the power efficiency were improvedin comparison with the comparative examples 31 and 33. This ispresumably ascribable to a fact that the temperature at evaporation waslowered by the use of a mixture, thereby reducing the evaporationtemperature and improving the film quality.

Comparative Example 34

As a next comparative example, a device was prepared by mixing andevaporating an Ir complex G (next structural formula) and the Ir complexC.

(Ir Complex G)

The Ir complex G has a light emission peak at 514 nm, while the Ircomplex C has a light emission peak at 620 nm, so that the lightemission spectra show little overlapping as shown in FIG. 3. In thepresent comparative example, the current efficiency and the powerefficiency are inferior to those of the example 3. This fact indicatesthat the device characteristics can be improved if the overlappingportion of the light emission wavelength of each light emitting materialis larger than the non-overlapping portion.

TABLE 9 Characteristics at luminance 100 cd/m² Light Current Poweremitting efficiency efficiency material Voltage (V) (cd/A) (lm/W) Comp.ex. Ir complex C + 6.3 10.5 5.4 34 Ir complex G Example 3 Ir complex C +5.7 13.5 7.6 Ir complex D

Example 4

In this example, there was employed a device configuration with afour-layered organic layer as shown in FIG. 1C. An ITO film (transparentelectrode 14) of a thickness of 100 nm was patterned on a glasssubstrate (transparent substrate 15).

On thus prepared ITO substrate, following organic layers and electrodelayers were formed in succession by vacuum evaporation by resistanceheating in a vacuum chamber of 10⁻⁴ Pa. In the light emitting layer,there were used plural light emitting materials:

Hole transport layer 13 (40 nm): FL03Light emitting layer 12 (40 nm): host material+light emitting material1+light emitting material 2+light emitting material 3+light emittingmaterial 4.Electron transport layer 17 (50 nm): BphenElectron injection layer 16 (1 nm): KFMetal electrode layer (100 nm): AlIt was so patterned that the electrodes had an opposed area of 3 mm².

In the present example, the Ir complex C was employed as the lightemitting material 1, and the Ir complex D was employed as the lightemitting material 2. An Ir complex E (following structural formula) wasemployed as the light emitting material 3:

Also an Ir complex F (following structural formula) was employed as thelight emitting material 4:

The Ir complexes C, D, E and F were mixed in a ratio of 3:1:2.5:3.5 toobtain a powder mixture. Thus obtained powder mixture was charged in anevaporation boat and co-evaporated with CBP as the host material. Thefilm formation was so executed that the above-mentioned Ir complexmixture represented 7 wt. % of the host material. The characteristics ofthus prepared device are shown in the following table 10, together withthe evaluation results thereof.

TABLE 10 Characteristics Luminance at luminance half life 100 cd/m² (hr)at Light Current Power initial emitting Voltage efficiency efficiencyluminance material (V) (cd/A) (lm/W) 1000 cd/m² Example 4 Ir 4.2 11.7 8116 complexes C, D, E, F Comp. Ir 5 7.6 4.8 50 ex. 31 complex C Comp. Ir7.5 6.8 2.9 5.4 ex. 32 complex D

In the device of the present example, the drive voltage required forlight emission at 100 cd/m² was 4.2 V, which corresponds to asignificant improvement in comparison with 5 V in the comparativeexample 31 and 7.5 V in the comparative example 32. Also the currentefficiency (measured in cd/A) was 11.7 cd/A, which was significantlyhigher than 7.6 cd/A in the comparative example 31 and 6.8 cd/A in thecomparative example 32. The situation was similar also in the powerefficiency, whereby a highly efficient device could be obtained.

Furthermore, the half life of the luminance in the continuous drive ofthe device from an initial luminance of 1000 cd/m² was 116 hours,corresponding to a significant improvement in comparison with 50 hoursin the comparative example 31 and 5.4 hours in the comparative example32. The half life of the luminance attained an improvement of from 2 toover 20 times in comparison with the comparative example, and isconsidered to represent a particularly large effect of the presentinvention.

Also the electric current supplied to the evaporation boat of thepresent example was lower in the mixture, indicating a lower temperatureat the film formation of the light emitting layer and a less thermaldamage. These results are shown in the following table.

TABLE 11 Light emitting material Boat current (Amp) Ir complex C 56.1 Ircomplexes C, D, E, F 55.7

Example 5

In this example, there was employed a device configuration with athree-layered organic layer as shown in FIG. 1B. An ITO film(transparent electrode 14) of a thickness of 100 nm was patterned on aglass substrate (transparent substrate 15).

On thus prepared ITO substrate, following organic and electrode layerswere formed in succession by vacuum evaporation by resistance heating ina vacuum chamber of 10⁻⁴ Pa.

Hole transport layer 13 (40 nm): FL03Light emitting layer 12 (40 nm): host material+light emitting material1+light emitting material 2Electron transport layer 17 (50 nm): BphenMetal electrode layer (100 nm): AlIt was so patterned that the electrodes had an opposed area of 3 mm².

In the present example, a compound C (abbreviated as DCM) was employedas the light emitting material 1.

As the light emitting material 1, there may also be employed a compoundD represented by the following structural formula:

The light emitting material 2 was composed of the aforementioned Ircomplex C.

The compound C and the Ir complex C were measured in equal amounts andwere agitated and mixed under crushing of the crystals in an agatemortar to obtain powder mixture. Thus obtained powder mixture wascharged in an evaporation boat and was subjected to co-evaporation withCBP as the host material.

The co-evaporation with the host material was conducted in such a mannerthat the aforementioned mixture of the compound C and the Ir complex Crepresented 7 wt. %.

Example 6

In this example, there was employed a device configuration with athree-layered organic layer as shown in FIG. 1B. An ITO film(transparent electrode 14) of a thickness of 100 nm was patterned on aglass substrate (transparent substrate 15). On thus prepared ITOsubstrate, following organic and electrode layers were formed insuccession by vacuum evaporation by resistance heating in a vacuumchamber of 10⁻⁴ Pa:

Hole transport layer 13 (40 nm): FL03Light emitting layer 12 (40 nm): host material+light emitting material1+light emitting material 2Electron transport layer 17 (50 nm): BphenElectron transport/injection layer 16 (1 nm): KFMetal electrode layer (100 nm): AlIt was so patterned that the electrodes had an opposed area of 3 mm².

In the present example, the Ir complex C was employed as the lightemitting material 1, and an Ir complex H (following structural formula)was employed as the light emitting material 2:

The Ir complex C and the Ir complex H were measured in equal amounts andwere agitated and mixed under crushing of the crystals in an agatemortar to obtain powder mixture. Thus obtained powder mixture wascharged in an evaporation boat and was subjected to co-evaporation withCBP as the host material.

The co-evaporation with the host material was conducted in such a mannerthat the aforementioned mixture of the Ir complexes represented 7 wt. %.

Comparative Example 61

A device was prepared utilizing the Ir complex C only as the lightemitting material, and executing co-evaporation with the host materialin such a manner that the light emitting material represented 7 wt. %.

Comparative Example 62

A device was prepared utilizing the Ir complex H only as the lightemitting material, and executing co-evaporation with the host materialin such a manner that the light emitting material represented 7 wt. %.

The results of evaluation of these devices are shown in the followingtable.

TABLE 12 Characteristics Luminance at luminance half life 100 cd/m² (hr)at Light Volt- Current Power initial emitting age efficiency efficiencyluminance material (V) (cd/A) (lm/W) 1000 cd/m² Example 6 Ir complex C +5.7 10.5 5.9 80 Ir complex H Comp. ex. Ir complex C 5 7.6 4.8 50 61Comp. ex. Ir complex H 5.8 16.2 8.8 1.5 62

In the device of the present example, the drive voltage required forlight emission at 100 cd/m² was 5.7 V, which was somewhat higher than 5V in the comparative example 61 but was significantly lower than 5.8 Vin the comparative example 62. Also the current efficiency was 10.5cd/A, which corresponds to a significant improvement in comparison with7.6 cd/A in the comparative example 61. The situation was similar alsoin the power efficiency, and the device of the present example was veryefficient with a power efficiency of 5.91 m/W in comparison with 4.81m/W in the comparative example 61.

Furthermore, the half life of the luminance in the continuous drive ofthe device from an initial luminance of 1000 cd/m² was 80 hours,corresponding to a significant improvement in comparison with 50 hoursin the comparative example 61 and 1.5 hours in the comparative example62.

The present example was inferior to the comparative example 62 in thecurrent efficiency and the power efficiency, but was improved in thelight emission color toward red in comparison with the comparativeexample 62. More specifically, the present example had CIE coordinatevalues of (0.68, 0.33) in comparison with the values (0.65, 0.35) of thecomparative example 62. The values of light emission of the comparativeexample 61 were (0.68, 0.33) and were almost same as those of thepresent example. The half life of the luminance attained an improvementof more than 10 times in comparison with the comparative example 62, andis considered to represent a particularly large effect of the presentinvention.

In the iridium complexes employed in the present example, and the HOMOlevel of the Ir complex C was −5.13 eV and was higher than the HOMOlevel of −5.19 eV of the Ir complex H.

On the other hand, the LUMO level of the Ir complex C was −2.47 eV andwas higher than the LUMO level of −2.6 eV of the Ir complex H.

The Ir complex C and the Ir complex H, employed in the present example,have the evaporation temperature in vacuum of 267° C. and 230° C.respectively, and, in general, the evaporation temperature is lower inthe iridium complex including fluorine atoms. One of the features of thepresent invention lies in a fact that the evaporation temperature can belowered and the cluster size can be made smaller at the evaporation byevaporating a mixture of light emitting materials of differentevaporation temperatures from a same crucible.

Also the electric current supplied to the evaporation boat was lower inthe mixture, indicating a less thermal deterioration at the devicepreparation.

These results are shown in the following table.

TABLE 13 Efficiency Boat current Light emitting material (cd/A) (Amp) Ircomplex C 6.5 56.1 Ir complex C + Ir complex H 10 52.6

Example 7

In this example, there was employed a device configuration with afour-layered organic layer as shown in FIG. 1C, with conditionsdescribed in the example 1.

A device was prepared by employing CBP as the host material and dopingthe light emitting layer with the Ir complex B as the light emittingmaterial at a concentration of 7 wt. % and with the compound A as thenon-light emitting compound at a concentration of 3 wt. %. The employedcompound A has a function of increasing the current in the device, thusalso serving as the current enhancing material.

Comparative Example 71

A device was prepared as in the example 7, except that the doping withthe compound A was not executed.

Following table shows the results of measurement of current andluminance of these devices under the application of a DC voltage of 10V.

TABLE 14 Current (mA/cm²) Luminance (cd/m²) Example 7 50.1 386 Comp. Ex.71 36.3 312

Table 14 indicates that the device of the example 7 showed increases inthe current and in the luminance in comparison with that of thecomparative example 71, thus confirming the effect of addition of thenon-light emitting compound and the effect as the current enhancingmaterial thereof. The light emission spectrum was almost same for theexample 7 and the comparative example 71, and the light emission wasobserved only from the Ir complex B.

Example 8

A device was prepared by employing TAZ as the host material and dopingthe light emitting layer with CBP as the non-light emitting compound ata concentration of 10 wt. % and with the Ir complex B as the lightemitting material at a concentration of 7 wt. %. The employed CBP has afunction of increasing the current in the device, thus also serving asthe current enhancing material.

Comparative Example 81

A device was prepared as in the example 8, except that the doping withCBP was not executed.

Following table shows the results of measurement of current andluminance of these devices under the application of a DC voltage of 10V.

TABLE 15 Current (mA/cm²) Luminance (cd/m²) Example 8 6.56 140 Comp. Ex.81 3.18 99.4

Table 15 indicates that the device of the example 8 showed increases inthe current and in the luminance in comparison with that of thecomparative example 81, thus confirming the effect of addition of thenon-light emitting compound and the effect as the current enhancingmaterial thereof. The light emission spectrum was almost same for theexample 8 and the comparative example 81, and the light emission wasobserved only from the Ir complex B.

CBP has a band gap of 2.5 to 3.0 eV, which is larger than that of 2 eVof the Ir complex B.

Example 9

There was prepared a device similar to that of the example 3 except forthe configuration of the light emitting layer. The light emitting layerwas composed of a mixture of a host material, a light emitting materialand a non-light emitting compound.

In the present example, the Ir complex C including phenylisoquinoline asthe ligand was employed as the light emitting material, and a compound 3(following structural formula) was employed as the non-light emittingcompound:

The Ir complex C and the compound 3 were measured in equal amounts andwere agitated and mixed under crushing of the crystals in an agatemortar to obtain powder mixture. Thus obtained powder mixture wascharged in an evaporation boat and was subjected to co-evaporation withCBP as the host material which was charged in another evaporation boat.The co-evaporation of the mixture of the Ir complex C with the hostmaterial was conducted in such a manner that the aforementioned mixtureof the compound C represented 20 wt. %.

The electric current supplied to the heating container at theevaporation of the mixture of the present example was found to be lower,whereby the evaporation temperature could be significantly reduced. Thisfact alleviated the thermal damage at the preparation of the device,thereby enabling stable preparation of the device.

The HOMO and LUMO levels of the iridium complex and the compound 3employed in the present example are shown in the following table.

TABLE 16 Ir complex C Compound 3 HOMO −5.13 −5.38 LUMO −2.47 −1.94 Bandgap 2.66 3.44

It will be understood that the Ir complex C constituting the lightemitting material has a band gap narrower than that of the non-lightemitting compound 3.

Example 10

There was prepared a device similar to that of the example 3 except forthe configuration of the light emitting layer. The light emitting layerwas composed of a mixture of a host material, a light emitting materialand a non-light emitting compound.

In the present example, the compound C was employed as the lightemitting material, and the aforementioned compound 3 was employed as thenon-light emitting compound.

The compound C and the compound 3 were measured in equal amounts andwere agitated and mixed under crushing of the crystals in an agatemortar to obtain powder mixture. Thus obtained powder mixture wascharged in an evaporation boat and was subjected to co-evaporation withCBP as the host material. The co-evaporation was conducted in such amanner that the mixture of the compound C and the compound 3 represented7 wt. %.

Example 11 Comparative Example 71

In this example, there was employed a device configuration with afour-layered organic layer as shown in FIG. 1C, and the conditionsdescribed in the example 1 were employed for the device configurationother than the light-emitting layer.

A device was prepared by employing CBP as the host material and dopingthe light emitting layer with the Ir complex B as the light emittingmaterial at a concentration of 7 wt. % and with PBD represented by thefollowing structural formula as the current enhancing material at aconcentration of 3 wt. %.

Also a device was prepared as in the example 11, except that the dopingwith PBD was not executed (comparative example 71).

Following table shows the results of measurement of current andluminance of these devices under the application of a DC voltage of 10V.

TABLE 17 Current (mA/cm²) Luminance (cd/m²) Example 11 62 450 Comp. Ex.71 36.3 312

Table 17 indicates that the device of the example 11 showed increases inthe current and in the luminance in comparison with that of thecomparative example 71, thus confirming the effect of the currentenhancing material. The light emission spectrum was almost same for theexample 11 and the comparative example 71, and the light emission wasobserved only from the Ir complex B.

In this case, the host material CBP has a strong hole transportingproperty, and doping with an electron transporting material such as PBDis effective for increasing the device current.

As the current enhancing material employable in the present invention,there may also be employed, for example, an electron transportingmaterial such as PySPy represented by the following structural formula,but such example is not restrictive.

Example 12 Comparative Example 81

In this example, there was employed a device configuration with afour-layered organic layer as shown in FIG. 1C, and the conditionsdescribed in the example 1 were adopted in the device configurationother than the light emitting layer.

A device was prepared by employing TAZ as the host material and dopingthe light emitting layer with the Ir complex B as the light emittingmaterial at a concentration of 7 wt. % and with NPD represented by thefollowing structural formula as the current enhancing material at aconcentration of 3 wt. %.

Also a device was prepared as in the example 12, except that the dopingwith NPD was not executed. This configuration was same as that of thecomparative example 81.

Following table shows the results of measurement of current andluminance of these devices under the application of a DC voltage of 10V.

TABLE 18 Current (mA/cm²) Luminance (cd/m²) Example 12 82 180 Comp. Ex.81 3.18 99.4

Table 18 indicates that the device of the example 12 showed increases inthe current and in the luminance in comparison with that of thecomparative example 81, thus confirming the effect of the currentenhancing material. The light emission spectrum was almost same for theexample 12 and the comparative example 81, and the light emission wasobserved only from the Ir complex B.

In this case, since the host material TAZ is an electron transportingmaterial, doping with a hole transporting material such as NPD iseffective as the current enhancing material.

As the current enhancing material employable in the present invention,there may also be employed, for example, a hole transporting materialsuch as m-MTDATA represented by the following structural formula, butsuch example is not restrictive.

In the foregoing examples, there have been explained cases employing ahost material, but the present invention provides similar effects alsoin a case not including the host material.

In summary of the foregoing results, the luminance, efficiency and lifetime of the light emission were improved in comparison with a case notincluding the non-light emitting compound.

POSSIBILITY OF INDUSTRIAL APPLICATION

As explained in the foregoing, the present invention enables to increasethe current flowing in a light emitting device, to drive such devicewith a lower voltage, and to improve the luminance and the lightemission efficiency.

The highly efficient light emitting device of the present invention isapplicable to products requiring energy saving or a high luminance.Examples of such application include a display apparatus, anillumination apparatus, a light source of a printer and a back light ofa liquid crystal display. In the application to the display apparatus,there can be realized a flat panel display of a low energy consumption,high visibility and a light weight. Also in the application to the lightsource of a printer, the laser light source currently employed in thelaser beam printer can be replaced by the light emitting device of thepresent invention. In such case, an image is formed by arrangingindependently addressable elements in an array and by giving an exposureof a desired form to a photosensitive drum. The use of the device of thepresent invention allows to significantly reduce the volume of theentire apparatus. Also in the illumination apparatus or in the backlight, there can be expected the energy saving effect of the presentinvention.

1.-31. (canceled)
 32. A light-emitting device comprising: a pair ofelectrodes; and at least an organic light-emitting layer disposedbetween the of electrodes, wherein the organic light-emitting layerincludes a first dopant for emitting light and a second dopant differentfrom the first dopant for transmitting energy to the first dopant, thefirst and second dopants being metal organic complex compounds eachhaving the same center metal.
 33. A light-emitting device according toclaim 32, wherein the difference of the peak wavelengths of the lightemission spectra of the first and second dopants does not exceed 30 nm.34. A light-emitting device according to claim 32, wherein the firstdopant is a phosphorescent dopant.
 35. A light-emitting device accordingto claim 32, wherein the second dopant is a phosphorescent dopant.
 36. Alight-emitting device according to claim 32, wherein each of the centermetals is iridium.
 37. A light-emitting device according to claim 32,wherein the first dopant has an isoquinoline skeleton.
 38. Alight-emitting device according to claim 37, wherein the second dopanthas an isoquinoline skeleton.
 39. A light-emitting device according toclaim 32, wherein the amount of the first dopant is greater than theamount of the second dopant in the organic light-emitting layer.
 40. Adisplay apparatus having a display section comprising the light-emittingdevice of claim
 32. 41. An illumination apparatus comprising thelight-emitting device of claim
 32. 42. A liquid crystal displayapparatus having a back light comprising the light-emitting device ofclaim 32.